Conventionally, in order to prevent occurrences of voltage saturations in AC servomotors, the following measures have been taken. That is, a negative d-axis current Id is supplied to an armature to which a dq conversion has been performed, while a d-axis direction is defined as a direction of field flux (refer to, for example, Patent Document 1). In an AC servomotor, a q-axis current Iq corresponds to an active current which is supplied in order to generate rotation torque, whereas Id corresponds to a reactive current which does not contribute to the generation of the rotation torque. However, because the current Id is supplied, an influence caused by back electromotive force produced in an armature can be reduced, and thus, a larger active current Iq can be supplied. As a result, current and torque control operations can be carried out stably.
This fact is well represented in FIG. 2 and FIG. 9 of Patent Document 1. These drawings indicate that an armature voltage is divided into 2 orthogonal voltage components, namely, a d-axis voltage Vd and a q-axis voltage Vq. A total vector summation between Vd and Vq is equivalent to a synthetic voltage in the armature. A circle shown in the drawing represents a link voltage, and this link voltage defines an upper limit value of the armature voltage. As a consequence, a desirable armature voltage is generated in accordance with the indication of this drawing as long as a tip portion of an armature voltage vector is present within the link voltage circle. Conversely, if the tip portion of the armature voltage vector is derived outside the link voltage circle, then the desirable voltage is not produced, and the desirable q-axis current Iq for generating the toque cannot be supplied (namely, so-called voltage saturation).
Next, in the respective drawings, back electromotive force E corresponds to a vector of a +q-axis direction, and concretely speaking, is expressed by such an equation E=ω·Φ which is well known in a synchronous type AC servomotor. In this equation, ω shows a rotation angular velocity of the motor, and Φ indicates synthetic magnetic flux which is intersected with an armature winging. The following description is made under such an assumption that ω≧0 and Φ≧0 are established, unless a specific comment is made. As previously described, since the back electromotive force E is directly proportional to ω, this back electromotive force E becomes larger during high-speed rotation. In FIG. 9, when the back electromotive vector E is increased, a tip portion of this vector is approached to the circumference of the link voltage circle, so that a large current Iq cannot be supplied. This reason is because, if Iq is increased, then a drive voltage vector of a +q-axis direction (same direction as E) is prolonged, so that the tip portion of the armature voltage vector is derived from the link voltage circle, and therefore, a voltage saturation occurs. The drive voltage vector is expressed as R·Iq in this drawing.
However, in this case, as shown in FIG. 2, if a reactive current Id (≦0) is supplied, then the difficulty is solved. This reason is given as follows. That is, by supplying Id, such a canceling voltage vector−ω·L·|Id|(−q-axis direction) can be generated which is directed opposite to the back electromotive vector E. Since this canceling voltage vector of the −q-axis direction is added, even when the above-mentioned drive voltage vector R·Iq of the +q-axis direction is prolonged, a tip portion of an armature voltage vector functioning as a summation of those vectors can be held within the link voltage circle. As a consequence, in accordance with this method, the large current Iq can be continuously supplied even during the high-speed rotation, and thus, large torque (∝Iq) can be stably generated.
[Patent Document 1] JP-A-9-84400 (pages 2 to 4, FIGS. 2 and 9)